Rex refers to the sub-label or series under Numero Group dedicated to reissuing and curating music that has a more popular or commercially successful past compared to the more obscure material typically handled by Numero Group. The Rex series might feature artists or bands that had moderate success during their time but have since faded into relative obscurity, making their works ripe for rediscovery and reappreciation.

The Numero Group's approach to music curation under the Rex series often involves remastering original recordings, packaging them with rare photos, and including detailed historical essays, making the reissues valuable not just for the music but also for their archival quality.